docs: def _get_site_amplification(self, sites, C):
          """
          Compute the fourth term of the equation 3:
          The functional form Fs in Eq. (1) represents the site amplification and
          it is given by FS = c61*S + c62*SS , where c61 and c62 are the
          coefficients to be determined through the regression analysis,
          while S and SS are dummy variables used to denote NEHRP site category
          C and D respectively
          Coefficents for categories A and B are set to zero
          """
          S, SS = self._get_site_type_dummy_variables(sites)
  
          return (C['c61'] * S) + (C['c62'] * SS)